1. Field of the Invention
The present invention relates to geographical coverage of wireless communication services. In particular, the present invention relates to a method for creating a map charting availability of wireless services and from which to identify coverage gaps.
2. Discussion of the Related Art
Access to a wireless communication system (e.g., a cellular telephone system) is typically provided by locating base stations of adequate capacity throughout the geographical service area. In a cellular telephone system1, for example, after a mobile unit (e.g., a handset) registers with a base station, all communications to and from the mobile unit are conducted over a wireless link with that base station until either the mobile unit leaves the operating range of the base station, or the call is terminated. The call from the mobile unit may be a voice transmission, a data transmission, or both. In the registration process, the mobile unit and the base station exchange identification information. The mobile unit's identification information is sent by a backhaul network to a control center of the wireless communicate system for authentication. The authentication process verifies that the mobile unit is authorized to use the base station, either as a subscriber of the carrier operating the wireless communication system (“host carrier”), or as a subscriber of another carrier having a roaming agreement with the host carrier. The control station typically tracks the mobile unit as the mobile unit moves from within the range of one base station into the range of another base station. 1 Examples of cellular telephone systems to which the present invention is applicable include CPDD, GSM, iDen, CDMA, GPRS, EDGE, and EvDO.
While communicating with a base station over a wireless link, a mobile unit typically monitors the quality of the wireless link. Examples of quality metrics of a wireless link include signal strength (e.g., signal-to-noise ratios) and bit error rates. The quality of the wireless link can vary with the distance of the mobile unit from the base station and is impacted by a variety of factors, such as the local weather conditions, topography of the surrounding area, and power of the cellular tower antenna. If the quality of the wireless link deteriorates below an acceptable level, or when another base station can provide a higher quality link—as when the mobile unit moves into the range of a closer base station—the registration of the mobile unit is transferred from the existing base station to the closer base station (“handed off”) under control of the control center.
A coverage gap occurs at any position within the geographical service area when an acceptable quality wireless link cannot be established between a mobile unit and a base station from that position. When a mobile unit moves into a coverage gap while a call is in progress, the call is involuntarily terminated. Such a condition leads to user dissatisfaction and the carrier also loses revenue. Thus, carriers devote significant resources to search for and identify such coverage gaps. As coverage gaps often result from local conditions (e.g., existence of barriers causing signal shielding), coverage gaps are identified often only by sending technicians out to the field at considerable expense.
In recent years, global positioning system (GPS) technology has been widely used in mobile assets or resource management applications (e.g., automatic vehicle location (AVL) and fleet management applications for the trucking industry). FIG. 1 shows one example of a location-based information system suitable for such applications. As shown in FIG. 1, a mobile unit located within vehicle 10-1 uses a GPS receiver to determine vehicle 10-1's geographical position based on GPS positioning signals received from GPS satellites 2 within its line of sight. The geographical position and selected operational parameters of the vehicle (e.g., fuel level, velocity, engine temperature, time since last engine ignition) are transmitted to a database managed by service computer 6. The transmission to the database can be via a wireless link over wireless communication network 8 or via another communication channel. Service computer 6 may be, for example, a server on the Internet. In this example, wireless communication network 8 may be connected to the Internet through a network gateway, as is familiar to those skilled in the art. Service computer 6 may include an interface to allow other users (e.g., fleet managers in the back office) to access its database from anywhere on the Internet.
Typically, vehicle 10-1 does not transmit its position and operational data continuously, but upon the occurrence of specified events (e.g., engine ignition, engine shut-off, and idling time exceeding a predetermined time interval), on demand by the mobile unit operator, or at predetermined time intervals. Between transmissions, vehicle 3 accumulates the data in a local storage device, such as a volatile or non-volatile memory device. However, if the mobile unit moves into a coverage gap of the wireless communication system at the time transmission is required, the transmission must be postponed until a wireless link is available at a later time. Such an event is often also required to be logged by the mobile unit and reported at the later time when transmission is possible.